Right now, you can download the latest version of the Android M Developer Preview for select Nexus devices. People with the Nexus 5, Nexus 6, Nexus 9, and Nexus Player can go ahead and grab system images of the Developer Preview for their devices. Doing so is as simple as manually flashing the system images from the source link below.
This is the third and final version of the Android M Developer Preview before Google delivers Android 6.0 Marshmallow to consumers later in the year. Expect Marshmallow to remain relatively unchanged between now and its launch.
Source: Android Developers